As electronic devices are rapidly reduced in size and weight, the demand is growing for batteries as a power source of electronic devices, secondary batteries which are small and lightweight, have high energy density and further, are repeatedly chargeable and dischargeable should be developed. Also, owing to environmental issues such as air pollution and the increase in carbon dioxide, an early practical application in an electric vehicle is anticipated. Therefore, there is a demand for the development of superior secondary batteries which have characteristics such as high efficiency, high power, high energy density and light weight.
As a secondary battery which satisfies these demands, the secondary battery which employs nonaqueous electrolyte has been put to practical use. The battery has several times higher energy density than a conventional battery which uses aqueous solution electrolyte. For example, a long-life, 4-volt class nonaqueous electrolyte secondary battery has been put to practical use. It employs lithium-containing layered cobalt oxide (hereinafter, Co-based compound), lithium-containing layered nickel oxide (hereinafter, Ni-based compound) or spinel-type lithium manganese composite oxide (hereinafter, Mn-based compound) for its positive electrode, and employs carbon material or the like which can absorb and release lithium for the negative electrode.
Among them, Ni-based compound, characterized in that its amount of the lithium which can be absorbed and desorbed within a potential range practically used in the nonaqueous electrolyte secondary battery (3.0-4.3V vs. Li/Li+) is equal to or larger than the cases of Co-based compound and Mn-based compound, also thanks to its availability, has largely been developed, aiming at a high-capacity and low-cost battery.
As described in Japanese Patent Publication No. H10-092429A, Ni-based compound is inherently more difficult to form so as to produce in a large amount thereof having a homogeneous crystal structure, compared with Co-based compound, which has been widely adopted. Due to subsequent improvements, however, examples are recently reported in which Ni-based compound incorporated into an actual battery delivers excellent performance (see Journal of Power Sources 119-121 (2003) 859-864, 865-869).